One time programmable (OTP) memories are often used to store program code and other information. Among other benefits, the one-time nature of OTP memories prevent authorized program code from being modified or over-written with unauthorized program code. OTP memories may be implemented, for example, using are fusible links, antifuse or floating gate non-volatile memory technologies. Fusible links are metal or polysilicon wires that are “blown,” i.e., made to have higher resistance, by passing a high current through them. As a result, fusible links exhibit some amount of physical destruction of the metal or polysilicon wire. Fusible links are relatively large and require relatively high current to program. Antifuse is the partial physical destruction or degradation of a Metal Oxide Semiconductor (MOS) capacitor gate oxide dielectric by the application of a high voltage. A lower resistance conduction path is formed between the plates of the capacitor through the oxide dielectric. Antifuse technologies require relatively high voltage to program and do not scale well with Complementary Metal Oxide Semiconductor (CMOS) technologies. The thinner MOS gate oxides associated with current CMOS technologies do not program consistently in a reliable manor.
Floating gate non-volatile memory involves the injection of electrical charge onto the isolated or unconnected (floating) gate of a field-effect-transistor (FET). The accumulation of charge on the gate changes the threshold voltage of the transistor, which can be sensed during a read operation. Floating gate non-volatile memories are employed for “flash” non-volatile memories. Floating gate non-volatile memories require relatively high voltage and sometimes relatively high current to program. Furthermore, floating gate non-volatile memories usually require additional special CMOS processing, thereby increasing fabrication cost.
A need therefore exists for improved OTP memories that are small in size and can be programmed with low voltages and small current.